1. Field of the Invention
The invention relates to a forward-reverse switching device for a vehicle and, more particularly, to durability when the vehicle is being towed.
2. Description of the Related Art
A planetary gear type forward-reverse switching device is widely known. The forward-reverse switching device is provided for a vehicle drive train that transmits power from a driving force source to drive wheels. For example, Japanese Patent Application Publication No. 11-51147 (JP-A-11-51147) and Japanese Patent Application Publication No. 2005-308041 (JP-A-2005-308041) describe such a forward-reverse switching mechanism. FIG. 12 is a view that shows an example of a drive train 1 equipped with a forward-reverse switching device 2. The forward-reverse switching device 2 is formed of a double pinion type planetary gear set (planetary gear train) 8 as a major component as in the case of the forward-reverse switching mechanism described in JP-A-11-51147. In addition, FIG. 13A and FIG. 13B each show a nomograph that can represent the relative relationship among the rotational speeds of rotating elements in the planetary gear set 8 using straight lines. In each nomograph, ρ indicated as the relationship between vertical axes is a gear ratio ρ (=the number of teeth ZS of a sun gear S/the number of teeth ZR of a ring gear R) of the planetary gear set 8, δ1 is (the number of teeth ZS of the sun gear S/the number of teeth ZP1 of each pinion gear P1), and δ2 is (the number of teeth ZS of the sun gear S/the number of teeth ZP2 of each pinion gear P2). In addition, straight lines L7, L8 and L9 in the nomographs represent the relative relationship among the rotational speeds of the rotating elements. In the forward-reverse switching device 2, for example, the sun gear S is coupled to an engine 3 so that power is transmittable, a carrier CA is coupled to drive wheels 5 via an automatic transmission 4 so that power is transmittable; whereas the carrier CA and the sun gear S are selectively coupled to each other via a clutch C, and the ring gear R is selectively fixed to a non-rotating member 6 via a brake B.
In the thus configured forward-reverse switching device 2, as indicated by the straight line L7 in FIG. 13A, when the clutch C is engaged, the planetary gear set 8 is placed in an integrally rotatable state to form a forward power transmission path. Power is transmitted to rotating elements arranged adjacent to the drive wheels 5 with respect to the forward-reverse switching device 2 through the forward power transmission path in the same rotational direction as the rotational direction of the engine 3. On the other hand, as indicated by the straight line L8 in FIG. 13A, when the brake B is engaged, the planetary gear set 8 forms a reverse power transmission path. Power is transmitted to the rotating elements arranged adjacent to the drive wheels 5 with respect to the forward-reverse switching device 2 through the reverse power transmission path in the rotational direction reverse (opposite) to the rotational direction of the engine 3. In addition, when both the clutch C and the brake B are released, the forward-reverse switching device 2 places a power transmission path in a neutral state (power transmission cut-off state) in which power transmission is cut off.